The present invention relates to an antenna for use in mobile radio telephone applications.
A common feature of mobile radio telephone systems is the division of particular geographic service areas into smaller units known as cells. Within each cell, a group of relatively low power base stations provides RF communication services to mobile subscribers located within the particular cell. The type of antenna system selected for use within a cell is important for maximizing system efficiency and for providing a field pattern suitable for the particular geographic features of the coverage area. In a common configuration, for example, a cell is divided into six equal sectors. Each sector is served by a separate directional receive antenna having a radiation pattern closely resembling the sector shape. A single omni-directional transmit antenna typically serves the entire cell.
The configuration just described is well suited for open spaces where cells and cell sectors may be configured having a relatively uniform shape. In some case, however, geographic features or man made structures prevent radio signals from penetrating all areas within a cell, or cell sector. For example, RF signals have difficulty penetrating the inner reaches of tunnels, and mobile telephone service is frequently interrupted when a mobile subscriber enters a tunnel of any significant length. Downtown city streets are other locations where radio telephone antennas may have difficulty transmitting to all locations within a cell or cell sector. The tall steel structures of densely located high rise buildings can interfere with the field patterns of antennas which are mounted high above street level, thus interfering with phone transmissions to those subscribers located on the streets below.
In addition to field pattern concerns, mounting considerations often determine the applicability of a particular antenna design in certain special situations. For example, in some cases it is necessary to mount an antenna directly on or near a large planar metal surface. With most antenna designs a large conductive surface located near the antenna's radiating elements will distort the field pattern of the antenna, and create large standing waves on the transmission line feeding the antenna. Typical Voltage Standing Wave Ratios (VSWR) for antennas mounted directly to a conductive surface are in the range of 3:1 or 4:1 or even greater.
Providing uninterrupted mobile radio telephone coverage within long tunnels presents some of the more challenging design requirements for mobile telephone base antennas. To broadcast to or receive signals from all points within a long narrow tunnel, it has usually been necessary to provide a plurality of antennas and transceiver units for transmitting and re-transmitting signals to provide coverage throughout the tunnel.
Similarly, crowded city streets within urban centers pose many of the same problems for mobile radio telephone systems as do tunnels. The long narrow spaces defined by city streets running between high rise steel buildings have many attributes of a long narrow tunnel. The same field pattern and mounting concerns arise when mounting an antenna to the side of a steel building for purposes of providing coverage down the length of a narrow city street as when mounting an antenna to the steel inner surface of a tunnel.
Therefore, it would be desirable to provide a bi-directional antenna capable of being mounted directly to a large planar conductive surface without significant degradation in the field pattern of the antenna while maintaining a relatively low VSWR. It is further desired that such an antenna be constructed having a relatively low profile so as to not protrude significantly into the service area. Finally, a strong streamlined radome should be provided to protect the antenna components and improve the visual characteristics of the antenna.